1. Field of the Invention
The present invention concerns an x-ray tomography apparatus for generation of multiple energy images and a method for generation of multiple energy images with such a tomography apparatus.
2. Description of the Prior Art
“Multiple energy images,” are x-ray images that are generated with different x-ray energies. For example, by a weighted subtraction on the basis of a low-energy image and a high-energy image it is possible to calculate images in which essentially only substances with a specific absorption characteristic are still visible. This fact is in particular used in diagnostic medicine for visualization of soft tissue separate from bony tissue. Multiple energy images can be generated in different manners.
In FujiFilm Co., Technical review no. 12, “Upright image reader that supports energy subtraction processing software”, an x-ray apparatus is described that enables the simultaneous generation of a low-energy image and a high-energy image with a fixed, set energy of the x-ray radiation by means of an energy-selective detector. The energy-selective detector has two detector arrays disposed in series, between which is arranged an absorption filter in the form of a copper filter for reduction of the energy of the x-ray radiation. Such detectors are very expensive to produce due to the use of two separate detector arrays. Moreover, with such a detector the achievable energy difference of the x-ray radiation used for generation of the high-energy image and of the low-energy image is very slight, such that bony tissue and soft tissue cannot always be completely separated in the calculated images.
Furthermore, a tomography apparatus is known from United States Patent Application Publication No. 2004/0101087 with which the low-energy image and the high-energy image can be generated via two separate exposures at different energies of the x-ray radiation. The different energies of the x-ray radiation are set by different voltages. The low-energy image is detected at a set voltage value of 80 kV and the high-energy image is detected at a set voltage value of 160 kV.
Given the same current values, dependent on the set voltage values the x-ray radiation exhibits a different x-ray dose or flux. Due to the equivalence of x-ray dose and flux, the following statements representative for both terms but only the term x-ray dose is used. Table 1 exemplarily shows the relation between the voltage and the relative x-ray dose. The x-ray dose is normalized relative to a set voltage value of 140 kV:
TABLE 1voltage vs. relative x-ray doseVoltageRelative x-ray dose140 kV100% 120 kV40% 80 kV20%
The x-ray dose at a voltage value of 140 kV (which is, for example, used for generation of the high-energy image) is accordingly approximately 5 times higher than the x-ray dose at a voltage value of 80 kV (which is used for generation of the low-energy energy). The patient is exposed to a different radiation exposure given the generation of high-energy images and low-energy images.